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For
most of the time there is a fairly uniform wind at high altitudes over
fairly wide geographical areas. It's strength is determined by the
prevailing atmospheric pressure distribution. By contrast, the wind near
ground level varies greatly according to the local topography, local
temperature variations, e.g. sea breeze, and the roughness of ground
features. The charts below give a simplified presentation of the average
energy in the wind over flat terrain. The source data was extracted from
the "Western Australian Wind Atlas".
Roughness Category
0
Open water areas.
1(a)
Open grassland within 1km of the coast.
1(b)
Open grassland inland (wheatfields).
2
Low vegetation 0.5 to 2 metres high.
3
Suburban areas, High vegetation > 2metres.
It will be seen from the
graphs on the previous page, that the proportionate increase in wind
energy from the use of tall towers, is greatest for those sites having the
highest roughness. A good rule of thumb is that the tower should be 2.5
times the height of any nearby buildings or trees.
The above graphs probably understate the case for
tall towers, since wind turbines are generally most efficient in the
middle of their power range. Thus there will be a compounding effect if
they operate for more of the time in this region. Also in inland regions,
the atmospheric boundary layer becomes thicker at night when there is no
mixing induced by the uneven heating of the ground by solar radiation.
With a tall tower, the wind turbine has more chance of penetrating the
boundary layer and intercepting a usable wind speed, when it is calm at
ground level. The chart below shows the relationship between efficiency
and wind speed for a 3kW wind turbine.
The theoretical limit for the efficiency of a wind
turbine, the "Betz" limit, is 59.3% but it is not achievable in
practice. On average, perhaps 25% of the kinetic energy intercepted by a
wind turbine rotor is converted to electricity.
The information in the charts on the previous page
refers to flat terrain. If a wind turbine can be sited on a hill, and
particularly on a ridge across the prevailing wind, the output is much
enhanced and there is less to be gained from the use of tall towers. The
wind speed at the top of an optimally shaped hill can be increased by as much
as 50%. Due to the energy being proportional to the cube of the wind
speed, this represents an energy increase by a factor of 3.4. The optimum
slope near to the top of the hill is about 16 degrees. The location of
wind turbines near to shear cliffs or quarry faces should however be
avoided, because such features induce severe turbulence which reduces
output and can reduce the life of the turbine blades.
From the information presented above it will be
seen that tall towers for wind turbines are likely to be cost effective at
all sites except those adjacent to the ocean, or on hilltops.
For example, the use of a
18m tower instead of a 12m tower for a Westwind 3kW wind turbine would add about 13% to the
installed cost but the output for inland sites in flat country would
increase by some 30% to 45% depending on the vegetation.
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